The present invention related electrical switches and more particularly to a surface mounted electrical switch usable in electrical test probes.
Electrical switches are generally enclosed packages having an exposed actuator mechanism permitting an external force to be applied to operate the switch. The actuator is connected to a pole contact, which engages or disengages fixed contacts in the switch during operation. The fixed contacts are connected to leads that extend from the package for connecting the switch to an electrical circuit.
Various types of electrical switch packages are being used in the electronics industry. DIP switch packages are multi-switch devices that are mounted directly onto a circuit board or the like. Some package types have multiple rocker type actuators engaging leaf-spring arm contacts within the switch to provide the switching function. Another package type is a multi-position switch having a sliding actuator containing a pole contact mounted on the underside of the actuator that selectively engages fixed contacts formed within the switch. Both type of DIP switch packages are enclosed within a plastic housing and have leads extending from the housing allowing the switch package to be connected to circuitry on the circuit board. The leads mate with through holes in the circuit board and are soldered into place. Examples of DIP switch packages are described in U.S. Pat. Nos. 4,031,345, 4,491,703, and 4,590,344.
Another type of electrical switch uses electrical contact pads formed on the circuit board as the fixed contacts of the switch. U.S. Pat. No. 4,013,885, assigned to the assignee of the present invention, describes a modular pushbutton switch that is removably mounted on a circuit board. The pole contact of the modular switch is attached to a pushbutton actuator that is secured in a insulative housing. The housing engages front and rear retaining members that are secured onto the circuit board containing the fixed contacts. The housing fits directly over the fixed contacts and movement of the actuator within the housing brings the pole contact into wiping engaging contact with the fixed contacts.
While the switches previously described are considered small devices occupying little area on a circuit board, the focus in the electronics industry is for smaller and smaller devices. As an example, surface mount technology has substantially reduced the size of integrated circuits and other type of electrical components. As a consequence, electronic test equipment manufacturers have been required to reduce the size of electrical test probes that are used to test these device and components. Most electrical test probes for electronic test equipment, such as oscilloscopes and the like, have an electrically conductive tubular outer body that is encapsulated in an insulating material. One end of the body contains a probing tip that is secured in the body using an insulating plug. A substrate is disposed within the tubular body and is electrically connected to the probing tip. The substrate, which may be a circuit board, has electrical components and circuit runs thereon proving probe compensation in relation to the input of the test equipment. The electrical circuitry on the circuit board and the tubular body are electrically connected to a transmission cable that is connected to the other end of the tubular body. The outer shielding conductor of the transmission line is generally connected to both the tubular body and to the circuit board to provide electrical ground coming from the test instrument. The center conductor is electrically connected to the circuit board for coupling the electrical signal present on the probing tip to the test instrument.
One type of electrical test probe in use today is a passive voltage probe. This type of probe has a compensated voltage divider network that presents a high resistive load to a device under test. Half of the voltage divider network is in the probe head while the other half is in the input to the test instrument. The voltage divider network may be configured to produce different levels of attenuation to an acquired signal. The most common attenuation factors used in passive voltage probes are one times (1.times.), ten time (10.times.), and one hundred times (100.times.). Generally, the input resistance to an oscilloscope is 1 megohm. Therefore, a 10.times. probe consists of a voltage divider network having 9 megohm resistor in the probe head and a 1 megohm resistor in the oscilloscope. In a 1.times. probe, the electrical signal from the device under test is directly coupled through the probe head to the oscilloscope input via the probe transmission cable.
One type of passive voltage probe is a combination 1.times./10.times. probe. A switch in the probe head routes the input signal either through the 9 megohm resistor in the probe head or directly to the transmission cable. The P6119 1.times./10.times. Switchable Passive Probe, manufactured by Tektronix, Inc., Beaverton, Oreg., the assignee of the present invention, is an example of such a switchable probe. The probe switch is an enclosed device having a rectangular shaped body that fits into a corresponding size hole in the probe head circuit board. Tabs extending outward from sides of the switch body are soldered to electrical contact pads formed on the circuit board. Internal to the switch, the tabs are coupled to fixed switch contacts. A sliding actuator in the switch has a pole contact formed on its bottom surface, which engages the various fixed switch contacts as the actuator is moved from one switch position to the next. The actuator is mechanically connected to a slide button on the outside of the probe through an aperture formed in the tubular outer body.
The diameter of the tubular outer body for this probe is substantially larger than the diameter of miniature passive probes used for probing surface mounted devices. This makes the probe difficult to use in probing such devices and circuits. To reduce the size of this probe to a miniature passive probe would require reducing the diameter of the tubular body. This would require surrounding the region of the circuit board containing the switch with an insulating material to meet the Underwriters Laboratories, UL, requirements for minimum through air and over surface distances between conductive elements and between conductive elements and a user for particular voltage ratings. For example, electrical equipment having a peak AC+DC voltage rating of 420 volts must have a minimum through air and over surface distance separation of 0.180 inches between conductive elements and between any exposed conductive element and a user. However, a portion of the insulating material would have to be removed to allow the mechanical connection between the switch actuator and the switch button on the outside of the probe. This modification would not maintain the minimum distance separation requirements set by UL.
What is needed is a switch design for a switchable passive probe that fits within the tubular body of a miniature passive probe while at the same time meeting the Underwriters Laboratories distance separation requirements for a 1.times. passive voltage probe capable of handling 420 volts peak AC.